1. Field of the Invention
The present invention relates to a variable compression ratio device, and more particularly to a variable compression ratio device that is capable of changing a compression ratio of a mixture in a combustion chamber corresponding to driving conditions of an engine.
2. Description of Related Art
Generally, thermal efficiency of combustion engines increases as the compression ratio thereof increases, and if ignition timing is advanced to some degree, thermal efficiency of spark-ignition engines increases.
However, if the ignition timing of the spark-ignition engines is advanced at a high compression ratio, abnormal combustion may occur and the engine may be damaged. Thus, the ignition timing cannot be advanced a large amount and accordingly engine output may deteriorate.
A variable compression ratio (VCR) apparatus changes the compression ratio of an air-fuel mixture according to a driving state of an engine.
The variable compression ratio apparatus raises the compression ratio of the air-fuel mixture at a low-load condition of the engine in order to improve fuel mileage. On the contrary, the variable compression ratio apparatus lowers the compression ratio of the air-fuel mixture at a high-load condition of the engine in order to prevent occurrence of knocking and improve engine output.
A conventional variable compression ratio apparatus can achieve a predetermined compression ratio of the air-fuel mixture according to a driving state of the engine, but it cannot achieve different strokes corresponding to intake/compression/expansion/exhaust strokes, respectively.
Particularly, if the expansion stroke is longer than that of the compression stroke, thermal efficiency may further improve. However, it is difficult to achieve a longer expansion stroke than compression stroke according to the conventional variable compression ratio apparatus.
In addition, a high compression ratio/low exhaust amount at a low load condition and a low compression ratio/high exhaust amount at a high load condition may be preferable in order to achieve low fuel consumption and high power output.
Herein, since oil pressure or an electric motor etc. must be provided as an actuator in order to change the compression ratio, a pump size is increased and electrical load is increased due to a large capacity electric motor.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.